I learned this the hard way during the SPG days when I thought I'd found a "clever" way to accelerate a mattress run. Back then you could still earn status based on stays rather than nights, so a rule like this made a bit more sense if the goal was to make it harder to earn status quickly, say, getting Platinum by booking 20 overlapping stays in a single week. The same is probably true of the practice of hotels combining multiple consecutive stays into a single stay, requiring you to bounce between properties or convince the front desk to leave them as separate stays. These rules became less relevant with the switch to night-based thresholds, but as we all know once Bonvoy takes something away, it never gives it back, so the rules stay on the books.